Non-oxide ceramics such as silicon nitride (Si3N4), silicon carbide (SiC), aluminum nitride (AlN), boron carbide (B4C), boron nitride (BN), and titanium carbide (TiC) are commercially important advanced ceramics for structural applications. Unlike, for example, the polishing of single crystal silicon nitride deposited on silicon (i.e., for microelectronic applications), the polishing of polycrystalline non-oxide ceramics are complicated by the physical properties of such material. For example, as a result of silicon nitride""s high hardness the best finishing processes generate roughness greater than 20 angstroms. However, further improvements in surface quality are hindered by preferential polishing and the heterogeneous removal of glassy-phase grain boundary materials. This complication results in raised grains with respect to the grain boundary phase. This invention provides a means for hindering this heterogeneous polishing effect by oxidizing the non-oxide ceramic grains during polishing. This process provides a equal removal rate for materials across the entire surface (with insitu oxidation during polishing and homogeneous removal).
The present invention includes the polishing of non-oxide ceramics using a chemo-mechanical polishing abrasive such as cerium oxide or a mechanical abrasive such as aluminum oxide, pH adjusted using nitric acid. Specific to nitric acid is a high rate of oxidation of almost any non-oxide material. As a direct result of this property, nitric acid is commonly employed in conjunction with other acids for the post-polishing etching of materials such as silicon.
The process at hand may include a first-step polishing stage in which surface and subsurface damage remnant from lapping is removed using any number of abrasive materials (aluminum oxide, cerium oxide, etc.). The polishing step utilizing the insitu oxidizing solution preferably incorporates pH adjusting the polishing abrasive to pH 2-4 using high concentrations of nitric acid (0.1-1.0 molar concentrations).
One aspect of the present invention is a method of finishing a polycrystal silicon nitride surface. The method of finishing the polycrystal silicon nitride includes providing a polycrystal silicon nitride composite which includes a plurality of silicon nitride crystal grains and a glassy-phase grain boundary medium. The method further includes providing an abrading finishing mixture with the finishing mixture including an abrasive and an oxidant. The method includes concurrently oxidizing the silicon nitride grains and abrading the silicon nitride composite to provide a finished polycrystal silicon nitride surface.
In another aspect, the present invention includes a method of forming a finished polycrystal silicon nitride surface. The method includes providing a polycrystal silicon nitride composite, which has a plurality of silicon nitride crystal grains and a glassy-phase grain boundary medium, and providing an abrading finishing mixture which has an abrasive and an oxidant. The method includes finishing the silicon nitride composite with the finishing mixture wherein the oxidant oxidizes the silicon nitride grains and the abrasive removes the glassy-phase medium and the oxidized silicon nitride grains wherein the resulting finished polycrystal silicon nitride surface has a roughness less than 10 angstroms.
In a further aspect the invention includes an abrasive finishing mixture for finishing a polycrystal composite having a glassy matrix and non-oxide crystal phases. Preferably the finishing mixture is a finishing slurry mixture. The finishing mixture is comprised of a plurality of abrasive particles and an oxidizer.
In another aspect the invention includes a method of polishing a non-oxide ceramic. The method of polishing includes providing a non-oxide ceramic comprised of a plurality of non-oxide crystals and a glassy phase boundary material. The method includes providing an abrasive finishing slurry mixture comprised of a plurality of abrasive particles and an oxidant, and finishing the non-oxide ceramic with the finishing slurry mixture to provide a polished surface.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.